All disk files require some means of determining the radial position of the read and write transducers (or heads) heads over the disks so that the heads can be accurately positioned over any desired track. Typically this is accomplished by recording head positioning information on one or more of the disk surfaces for reading by magnetic or optical heads. Some disk files, known as dedicated servo files, contain positioning information only on a dedicated surface of one disk in the disk stack. In contrast, some modern files, known as sector servo files, store the positioning information interspersed with the data on each disk surface. This latter approach represents the direction in which the technology is progressing and is preferred because it can be implemented at low cost and without additional componentry beyond that required to store the data and further because it provides the positioning information at the data surface being accessed, thereby eliminating many mechanical and thermal sources of track misregistration. Fixed block architecture (FBA) is a common configuration used to format both dedicated servo disk files and sector servo disk files. In an FBA formatted disk file, each disk track is divided into a number of equal-sized sectors, and each sector is divided into regions containing either positioning information or data.
A portion of a typical FBA sector servo sector is illustrated in FIG. 1A. The portion shown spans the end of one sector and the start of another, and shows in particular the positioning information provided between sectors for three adjacent tracks--A, B and C. 101 shows the end of data regions for tracks A, B and C. 102 shows the start of the succeeding data regions for tracks A, B and C. In between is head positioning information for each track designated generally by 103 and including position identification (position-ID) field 104 and position error indicating (PES) field 106. Position-ID field 104 typically includes a track number and/or a sector number to provide radial (track) and/or circumferential (sector) orientation for the recording head. It may be encoded in a gray format, in which the encoded numbers on adjacent tracks differ by only one bit. This allows the track/sector id to be known to.+-.1 tracks, even when inbetween tracks. PES field 106 provides a fine positioning signal used by the disk file to position the recording head precisely over the center of a given track.
In a dedicated servo disk file, the positioning information is encoded on a separate surface from the data surfaces. The positioning information is encoded in a manner similar to that described above with reference to sector servo disk files, and is similar in form to positioning information 103.
While the position-ID field provided by the prior art is designed to be accurately readable when the recording head is on-track, there are circumstances in which it is desirable to accurately read the position-ID information when the head may not be positioned on-track. Such circumstances may arise during a settle operation following a track access when the head has not yet settled over the desired track but it is nevertheless desired to read the position-ID field, or where the servo architecture depends on the servo system to precisely identify the sector number. Such circumstances may also arise in magneto-resistive (MR) head designs where one element is used for writing and another for reading. Misalignment between the read and write elements can occur due to production constraints (inability to precisely align the elements at manufacture time) and due to design constraints (inter-element shift as a function of accurate direction of access in rotary actuator systems).
The effects of misalignment can be alleviated by a technique known as micro-jog, which involves adjustment of the head-track position between reading and writing to allow the appropriate head element to be on-track for a given operation. However, for a write operation a micro-jog itself may move the read element significantly off-track.
In the above mentioned circumstances in which it is desirable to read positioning information when the head is off-track, the prior art position-ID field described above suffers increased error rate (percentage of READ operations that fail to detect the sought-after information) due to interference from the adjacent track and decreasing signal-to-noise ratios. This problem is somewhat correctable by encoding the position-ID field in gray code so that the position-ID can be known to within a value of.+-.1. However, when the head is off-track, the position where the single changing bit changes from a 0 to a 1 is not well defined. Also, the position-ID information is still susceptible to errors when read, so in cases where the position-ID must be known exactly, gray coding may prove ineffective. Moreover, the use of gray coding increases the length of the position-ID field. The values 0 and 1 must be recorded using the same number of magnetic transitions to preserve the magnetization state, effectively doubling the length.
FIG. 1B. shows a possible means for adding error handling information to the head positioning information. An error handling (EH) field 105' is appended to the position-ID field 104'. This field may be encoded in a number of ways, including a cyclic redundancy check (CRC), error checking and correction code (ECC), parity, or redundant encoding (writing one or more extra copies of the field). However, EH field 105', which is provided as an accuracy check against position-ID field 104', cannot be encoded as a gray code. If the ECC bits are merely appended to the gray code, as shown, they will also suffer an increased error rate when read off-track.
Thus, there presently exists an unmet need for a method and means which allows the positioning information to be accurately read off-track with no increase in error rate.